Monday, August 26, 2013

With just 5 days left until the release of my Official 2013-2014 Winter Forecast PLUS 20 selected cities' winter forecasts at noon central time, I'll shed a little light on where we currently stand in the atmosphere, and what implications it could have for this winter.

Let's start off with sea surface temperatures over the world's oceans. Sea surface temperatures are significant indicators to high and low pressure formation over certain areas, so we want to be paying close attention to these SST charts as we move on through fall and into winter. There are a few areas I want to point out for this SST chart. The first region of interest is the body of above normal sea surface temperatures across the northeast Pacific. Per recent developments around the Pacific, the negative PDO regime that has been holding a grip over synoptic weather in North America for years now, dissipated. In its place, we have strong +SST anomalies. I'm feeling pretty good about the placement of those +SST anomalies, in the sense that such warm temperature anomalies could provoke persistent high pressure formation over the west coast of North America, which would go hand in hand with my analog set for this winter, which also shows high pressure straddling the west coast of North America. This persistent high pressure along the west coast of North America could very well then provoke a positive PNA pattern, which would add to cold weather prospects for the Central and East US. But of course, this is all educated speculation.
Moving on to the temperature anomalies along the Atlantic waters stretching from the Canadian Maritimes directly west to Europe, we see +SST anomalies in this area. Just like the Pacific, these +SST anomalies could provoke high pressure anomalies over the Atlantic, and this is something I have seen in another analog set I published on this blog, and do not discount the idea of such a scenario unfolding. If we look north into Greenland and the bodies of water in Canada, we see a continued trend of positive SST anomalies. If these positive SST anomalies continue into the winter, we could see the chances of a negative NAO increase, which could then lead to increased instances of coastal storms along the Eastern Seaboard. However, these coastal storms could go to the Midwest if that Atlantic ridging pattern I discussed earlier works out and expands into the East Coast. Again, it's all educated speculation.

Next, we will take a look at stratospheric temperature anomalies. The graph above comes from the Japan Meteorological Agency, and shows 30 millibar temperatures (black line) superimposed on the average 30mb temperatures for that time of year (gray line). The trend this summer has been above normal stratospheric temperatures, and the 30mb level is included in this trend. The upper stratosphere is not really included in this above normal temperature trend, but the mid and lower stratospheric levels are, and it is these two regions of the stratosphere that hold the strongest influence on the weather down here in the troposphere. This is all good news for winter weather lovers, especially if the warm stratosphere trend can continue through the fall and into winter.

This image shows zonal-averaged winds at the 30 millibar level- this is the same level we discussed in the temperature chart above. However, this zonal wind graphic has a much different bearing on winter weather. Take a look at the blue line stretching along the far left side of this chart. See those blue colors? That is a trademark sign of a westerly (positive) QBO. The QBO involves the stratospheric zonal wind pattern, and can have strong impacts on winter weather. The positive phase of the QBO means the stratosphere is less inclined to experience sudden stratospheric warming (SSW) events. If you recall from this past January, SSW events can lead to Arctic outbreaks of cold air, if the pattern is right. So, if you like winter weather, you don't want to see a +QBO for the winter. But let's look a little closer at this +QBO. As you can see, the blue strip of positive zonal wind anomalies signifying the positive QBO has been present since roughly May, and has strengthened since mid July. But look at the zonal wind chart since then. If you look closely, you can see the blue strip slowly narrowing along the 10S line on the bottom legend of this chart. Looking even closer along the EQ line right at the end of the left side of the graphic, you can see the darkest blue color is starting to become raggedy, and not as strong as it was earlier in June and July. If you didn't really see what I'm talking about, this is the point: there are small signs that the positive QBO is weakening. This would be great news for trying to break out of the positive QBO pattern and getting into at least a neutral QBO, which would enhance chances for sudden stratospheric warming events that would have otherwise been hampered by the +QBO.

All in all, there's still a lot of time until winter comes around, but current trends are rather favorable for this winter in terms of cold weather for the Plains, Midwest and portions of the East US.

Remember: The Official 2013-2014 Winter Forecast comes out at 12:00 PM Central Time this Saturday, August 31.

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